Bats are small, flying mammals from the order Chiroptera. Of the 25 species found in California, almost all are insectivores that feed on vast numbers of night-flying insects, making them an important part of the ecological community.

Several of these species—pallid bat (Antrozous pallidus), big brown bat (Eptesicus fuscus), Yuma myotis bat (Myotis yumanensis) and other Myotis species, and Mexican free-tailed bat (Tadarida brasiliensis)—frequently use man-made structures such as attics, barns, or bat boxes for roosting sites and are the species you most likely will find in urban areas.

Bats have relatively long lives—5 to 30 years depending on the species—and are among the slowest reproducers for their size of any mammal. For example, the little brown bat, the most frequent user of artificial bat habitats in the United States, can live for 3 decades with the female giving birth to 1 pup per year.

In most cases, bats don’t cause problems for homeowners or gardeners, and because of their nocturnal habits, you rarely will see them. Because they eat insects, bats also might provide some control of insect pests in the landscape.

Problems that do occur often happen when migrating bats roost in buildings, usually during warmer months. Their droppings can accumulate, they can make noise, and some people are uncomfortable with close proximity. Bats also can transmit diseases, with rabies being a special concern.

BIOLOGY AND LIFE CYCLE

As mammals, bats have fur, give birth to living young, and provide their young with milk. In spring, female bats form colonies to give birth and rear young. Roosting sites include buildings, bridges, or other structures as well as caves or tree cavities. Adults leave roosts at night to forage for insects. Young bats develop rapidly, and most are able to fly within a month or two after birth. Generally males and females with young will roost separately, but in late summer or fall, males might join the colony, and bats might migrate to warmer areas or hibernation roosts in the winter when insects are scarce.

Bats are excellent flyers and navigate using echolocation to capture insects in flight during the night. Because bats consume large numbers of insects, some people have installed bat houses with the idea that colonizing bats might control mosquitoes or other pest insects.

PUBLIC HEALTH CONCERNS

Although bats provide benefits by feeding on insects, they also can carry diseases to humans and other animals. You can greatly minimize the risk of disease transmission by never handling bats, not breathing dust from bat droppings, and vaccinating your dogs and cats against rabies. It is very important to educate children never to touch a bat, dead or alive.

Bats also create unsanitary conditions when their droppings and urine accumulate beneath roosts, creating odors and attracting insects. More information regarding bats and disease prevention is available from the Centers for Disease Control and Prevention.

Rabies

Rabies is a viral infection of the central nervous system that causes inflammation of the brain, and sometimes the spinal cord, in humans and other mammals. Once symptoms appear, the disease is almost always fatal. A rabies exposure requires contact with an open wound, abrasion, or mucous membrane—such as the eyes, nose, or mouth—and a rabid animal’s saliva or nervous tissue. Non-bite exposure to the virus is extremely rare, and no evidence exists of transmission through contact with urine or feces.

Worldwide, more than 30,000 humans die from rabies each year, and 99% of these deaths are due to contact with rabid dogs. The threat of rabies is virtually nonexistent for anyone who vaccinates all family dogs and cats, avoids contact with unfamiliar animals, and never handles wild mammals. Any bat or other wild animal that you can catch is likely to be sick, so only an expert should handle them. Laboratory personnel, biologists, or other individuals who have a high probability of contact with infected animals can receive a series of pre-exposure vaccinations to simplify treatment if exposed.

Like all mammals, bats can contract rabies, but even rabid bats rarely bite except in self-defense. Potentially rabid animals that heath department personnel test usually are sick or injured and have had human or pet contact. In tests of randomly selected bats, no more than 0.50% have been rabid, and no evidence exists that rabies from bats ever has triggered an outbreak of rabies in other animals. However, the odds of a rabies infection in a sick bat—those most commonly handled by the public—are higher. Overall, the odds of being harmed by a bat are remote for those who don’t handle them.

If you are accidentally bitten while handling a bat, make sure you save the mammal for examination and contact your county health department. Immediately wash the bite with soap and water, and seek prompt medical advice. A non-bite exposure should be treated in the same manner as a bite. According to the Centers for Disease Control and Prevention, modern rabies treatment is highly effective and relatively painless. If disease symptoms appear, treatment no longer might be possible, which could lead to death, so it is important not to wait to seek medical advice from a physician or health authority.

Histoplasmosis

The fungus Histoplasma camsulatum capsulatum causes the respiratory illness histoplasmosis. The fungus occurs naturally in the soil in warm, humid areas, and bat and bird droppings enhance its development. Human infection occurs by inhaling the fungal spores that disperse when dry fecal deposits are stirred up. Bird roosts are the most important sources of the infection, and more than 99% of all cases in the United States are found in the Ohio and Mississippi river valleys. This disease rarely has been reported in California.

The fungus normally doesn’t survive in hot, dry attics. However, to reduce risk of exposure, persons cleaning up either bat or bird droppings should wear proper personal protective equipment including respirators that filter particles as small as 2 micrometers. In some situations a wetting solution can help keep dust down.

Bat Parasites

Ectoparasites are organisms that feed on other animals. Bat ectoparasites include fleas, flies, true bugs, chiggers, ticks, and mites. Most bat parasites are host specific and can’t survive on other animals. No evidence exists of disease transmission to humans or domestic pets from bat parasites. Parasites that remain after bats have been removed from buildings soon die without their hosts.

MANAGEMENT

Bats that you find on the ground or out in the open during the day aren’t always sick. If they are in an area where they aren’t harming anyone and pets and children aren’t likely to find them, leave them alone for an hour or two. Sometimes they’re tired from long migration flights.

If they are in an area where children or animals might find them, gently scoop the bat into an open box, wearing leather gloves to prevent touching the bat with your bare hands, and move it to a place where no one can come into contact with it. If the bat has been in contact with pets or people, take it to the county health department, so trained professionals can check it for rabies. Once a bat has died, it is too late to check for rabies, because the rabies virus dies shortly after the host does.

Sometimes a colony of bats will show up at a house in the spring or fall. Often this is a migratory colony, and it will move on after a few weeks of rest. If the bats are in an area that can be tolerated, such as an outside eave, then wait a few weeks, and once the bats have left, seal the area, so they can’t return.

Bats roosting in barns or other outdoor buildings might not cause any harm. Some people encourage bats by placing bat houses on their property. However, because of the threat of rabies and other diseases, bats should be discouraged from nesting at locations where people congregate, such as schools, parks, public buildings, or homes. Bats flying inside inhabited buildings should be removed or excluded.

If you suspect bats are roosting in your building, you’ll need to carefully look for signs of them. Bats can squeeze through openings as small as 1/4 inch. Cracks around windows, doors, pipes, electrical wiring, or vents can provide access. Being alert to off odors or to droppings beneath openings can help you locate roosts and entry points. Return late in the evening, just before dark, to observe how bats are gaining entry into a building by watching where they fly out. Exclusion is the primary way to manage bats, but you’ll want to be sure all bats have left the structure before you seal it up completely.

Most bats you discover indoors will be flying, but some might be roosting or asleep. During cool weather, bats can become torpid; this reduced activity is due to a lowering of their body temperature. Torpid bats might appear to be sick or dead or might bare their teeth or hiss, a defensive behavior to ward off potential predators. It often takes up to an hour or more for a torpid bat to warm enough to take flight. You can gently scrape a torpid bat into a can or box, cover the can, allow the bat to warm up in safety, then release it outside. Never handle bats unless you are wearing leather gloves, since a frightened animal is likely to bite in self-defense.

Removal from Dwellings

A bat that flies into human living spaces usually is a young, lost one, and its only goal is to escape. A sudden, panicked exit by humans probably is the worst possible action, as it can cause the bat to hide, making it not only difficult to find but almost impossible to notice if it manages to find a way out on its own.

The best action is to keep the flying bat in sight while closing doors to other parts of the house and isolating the mammal to a single room, where no pets or family members are present. If possible, open doors and windows to the outside, so the bat can find its own way of escape. As a further attempt, wait until the bat lands, and approach it slowly. Place a small box or coffee can over the bat, and gently slide a piece of cardboard beneath the container, trapping the bat inside. Then you can release the bat outdoors. Wear leather gloves to avoid being bitten.

As a last resort you might be able to catch a bat in flight using a fishing or butterfly net swung from behind the mammal. However, it sometimes is difficult to release bats from netted material. If this occurs, turn the net inside out, and let the bat try to work itself free. Avoid handling any bat unless you are wearing leather gloves.

Excluding Single Bats

Bats can enter through open doors or windows. Other common entry points include chimneys that don’t have grates and doors with loose-fitting screens. Placing 1/2-inch or smaller welded wire mesh over chimney tops (Check with your fire department for local regulations on spark arresters.) and installing tighter-fitting screen doors can solve these situations. Conduct a room-by-room search for other points of entry, and seal holes that are more than 1/2 inch in diameter or cracks that are 1/4 by 1–1/2 inches or larger. You can close openings around plumbing pipes by using steel wool or other suitable material. Bats don’t chew holes or gnaw electrical wires.

Excluding Bat Colonies

Large numbers of bats living in attics or walls are undesirable. Most bat colonies are small and go unnoticed for years, but when odor or noise increases or bat droppings accumulate, eviction and exclusion are the only safe and permanent remedies.

Entry points of large colonies usually are quite obvious. When bats start emerging at dusk to feed, watch the building to see where they exit. Daytime inspection should reveal the holes or cracks they are using. Often, these will be beneath eaves, around the chimney, around air and plumbing vents, near loose boards, beneath roof caps, or in openings that squirrels or other animals have made.

You also can identify the location by looking for staining caused by body oils (similar to rodent “rub marks”), bat urine, and bat droppings. Bat droppings, known as guano, crush easily, revealing shiny bits of undigested insects. Mouse droppings lack the shiny bits, and bat droppings never are white or chalky as are bird droppings. Seal entryways with caulking, putty, duct tape, self-expanding polyurethane foam, or hardware cloth (1/4-inch mesh) after you have excluded the bats.

You’ll want to avoid excluding bat colonies when flightless young could be present, usually May through September in California. Excluding the parents will starve the young and create odor problems and an infestation of flies. Most bats leave in late fall in most areas, making winter an ideal time to implement exclusion techniques.

If exclusion needs to be done before bats have moved out of a structure, you’ll need to provide a way for them to leave at night that doesn’t allow them to re-enter. A relatively simple technique involves hanging 1/2-inch polypropylene bird netting, or another type of fabric that allows light to show through, above the exit using duct tape or staples, so bats can still see their exit hole. One netting vendor is Wildlife Control Technology (www.wildlife-control.com), and bird netting is available at many hardware and garden stores. Attach the netting 12 inches above the exit holes, and extend it 12 inches past the sides and bottom. The bottom must hang freely, creating a one-way flap that allows the bats to drop down to leave.

Another method is to insert a 2-inch diameter plastic pipe with a tube of collapsible material, such as plastic tarp, attached to its end. When the bats return, they can’t fly back in. Be sure to seal up all other potential entryways.

Leave the netting or exit tube in place for 7 days to ensure no bats are trapped inside. Then you can make more permanent repairs.

You can exclude bats from Spanish tile roofs using bird netting, but the simplest solution often is to install rain gutters. You also can fill the open ends of the tiles with concrete or tightly fitted bird-stop strips. Mylar strips 2 inches wide by 10 inches long also can deter bats.

Chemical Repellents

No pesticides or chemical repellents are registered in California for controlling bats or repelling them from roosting areas.

Mechanical Repellents

Some mechanical repellents can help discourage bats. Illuminating attic spaces and eaves with electric lights 24 hours a day might be helpful. Cooling an attic with fans can make the temperature unsuitable for roosting, and the increased air movement also can create a less favorable roosting area. All ultrasonic sound generators that have been tested have proven to be ineffective for repelling bats.

BAT HOUSES

Some people are interested in encouraging bat populations by installing artificial roosting sites or bat boxes. Bat houses aren’t likely to discourage bats from roosting in nearby buildings, and there is no clear evidence that their installation will significantly reduce garden insect problems. If you chose to put up a bat house, make sure it is in an area that has minimal disturbance from people and animals. Sometimes sick bats fall out of the roost where children and household pets easily can pick them up.

Bat houses and bat-house designs are widely available commercially. Follow the guidelines below when constructing and installing a bat house:

Use rough-sided wood on the interior of the house. Horizontally groove the interior surface for toe holds.

Protect the house from prevailing winds, if possible, and provide an unobstructed approach for flying bats.

Bat houses attached to the sides of buildings tend to have the greatest success, but be careful, since bat droppings will accumulate below. Bat houses on poles in open areas sometimes work, but bats rarely occupy houses placed in trees. Installing bat houses before March improves the chance of occupancy. It is not unusual for bat houses to remain unoccupied for a year or more. Bats prefer houses that are within 1/4 mile of water with enough room to drink on the wing. More information on bat boxes is available in the UC ANR publication Songbird, Bat and Owl Boxes.

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